Will I keep my rank in the army reserves if I promoted after I signed the contract ?

No, you won't be demoted, but you will want to go back to your Reserve Component Career Counselor so that they can find you a spot in your unit with your new rank. This is especially important if you're getting a bonus. The bonus is tied to your specific slot in a unit and if you aren't filling that specific slot MOS/Grade and meet the MOS/Grade requirements of that slot, you will no longer be entitled to that bonus

If you have your promotion orders and info for your reserve unit CDR and S1, give them a heads up and provide a copy of the orders. It shouldn't be too difficult for them to adjust you on their UMR.